Effect of gypsum application rate and leaching reginle on \vheat 0To'wth in a highly acidic subsoil b C.D.A. McLay and G.S.P. Ritchie Department of Soil Science and Plam Nlltrition, School of Agriculture, University of Weslem Australia, Nedlal1ds, WA 6009, USA . Abstract A glasshouse experiment "vas conducted to investigate gypsum application and leaching on the amelioration of" an at uIl1inium (AI) toxic subsoil for wheat growth. Treatments included di ft'erent rates of gypsum application and amount of leaching prior to wheat being grown. Wheat shoot growth increased vvhen gypsum was applied in both the presence and absence of leaching, but growth was higher with leaching. Gypsum application led to a decrease in toxic Al as a result of a higher ionic strength and activity of A1501- ion pairs in the soil solution, and increased Al leached from the soil. Root growth may not be a good bioassay for predicting \vheat shoot responses to gypsum applications on acidic subsoils. Introduction GypSUIl1 applied to the soil surface has often been observed to be more effective than surface applied lime at improving crop yields on soils which have acidic subsoils (Shainberg et aI., 1989). The differ- ence in responses to lime and gypsum has general- ly been related to movement through the soil pro- file. Gypsum is rapidly leached to the depth of soil where acidity is limiting growth whereas lime move- ment into the subsoil is generally very slow. A number of mechanisms have been proposed by which gypsum may ameliorate aluminium toxicity in acidic subsoils (Shai nberg et a1., 1989), although the actual mccha- nism(s) which operate in different soils remains poorly understood. In some studies, it has been suggested that calci um (Ca) ions displaced AI ions ofT soil surfaces and therefore cause AI to be leached from the soil in the drainage water (Kotze and Deist, 1975; Oates and Caldwell, 1985). Other studies however have not reported changes in the concentration of total AI in the subsoil, and have suggested that an increase in the concentration of Ca or SO" ions decreases the toxicity of Al to plants. In \Vestern Australia, a large area of deep yellow sandplain soils, which occur in a region of low rainfall (c. 300 mm annum - l), have been shown to have toxic concentrations of AI which deCl"ease wheat yields (Carr et aI., 1991). If leaching of AI from the soil profIle was essential for gypsum to effect amelioration, it is uncertain whether there would be suflicicnt raineal! in the environment where yellow sandplain soils occur for gypsum to be crfective. The aim orthe experiment, therefore, was to establish whether leaching of Al was an essential mechanism for gypsum to decrease AI toxicity and improve wheat growth in the subsoil of a yellow sandplain soil. IYlaterials and methods A factorial pot experiment was designed with three gypsum rates (0 g gypsunl kg I - GO; 0.142 g gyp- sum kg- l - GI; 0.567 g gypsum kg-I -G2) and three leaching regimes (unleachcd - LO; 250 mm drainage (equivalent to one winter's rainfall) in five daily leach- ing events of 50 mIll each - L 1; and 500 Illm drainage in flve leaching events of 100 mm each -L2). Soil was incubated overnight at 40°C between each leaching